Cells related to the B lymphocyte lineage of mice were studied to determine their developmental relationships to other hematopoietic cell types and the contributions of oncogenes to tumor induction. B cell development: a wide variety of hematopoietic neoplasms were studied for the organization of immunoglobulin and T cell receptor genes to determine lineage relationships based on the activity of recombinases. Rearrangements of receptor genes were detected in T cell=B cell >> myeloid tumors but not in erythroleukemias, suggesting an early deviation of erythroid progenitors from precursors for the early deviation of erythroid progenitors from precursors for the other lineages. Close relations between the B cell and myeloid lineages are also suggested by the demonstration that B cell progenitor lines can differentiate to yield mature macrophages. Oncogenes: a new oncogene that induces pre-B cell lymphomas in vivo and transforms fibroblasts in vitro was cloned and sequenced. The sequence was unrelated to that of any previously described oncogene and it has been named v-cbl. Cellular sequences related to v-cbl were mapped to mouse chromosomes 6 and 9 and were designated c-cbl and c-cbl-2. Mice inoculated with retroviruses containing v-myc were found to develop clonal pancreatic acinar cell carcinomas that could be transplanted and established as in vitro cell lines. In contrast, mice inoculated with retroviruses containing c-myc developed myeloid tumors.